Seal apparatus

ABSTRACT

A seal ( 40 ) provides sealing between two pressure zones ( 60, 62 ) and a rotating and non-rotating member ( 42, 52 ) by providing a first sealing member ( 46 ) between two seal lands ( 48, 50 ). A second seal ( 64 ) is mounted between the non-rotating member ( 52, 58 ) and one of the seal lands ( 50, 66 ) so as to control pressure around the seal lands ( 48, 50 ).

[0001] This invention relates to seal apparatus. More specifically butnot exclusively this invention relates to a seal for sealing between onerotating member and one static member.

[0002] It is frequently necessary to seal a clearance gap between twocomponents that are capable of relative movement. In particular one ormore seals are often required to provide a seal between a rotatableshaft and an axially adjacent static component. For example, a gasturbine engine comprises shafts which rotate at relatively high speedsand which are exposed to pressurised hot gases. Seals are requiredbetween rotating rotor blades and surrounding static casing structure.

[0003] Seals are also required between a rotor carrying such rotorblades and an adjacent static structure which carries stator vanes ornozzle guide vanes. In a gas turbine engine nozzle guide vanes or statorvanes are non-rotating and as such mounted on a static structure.

[0004] It is also important to provide such seals to prevent hotpressurised gas in one pressure zone flowing freely into an adjacentlower pressure zone.

[0005] It is well known to provide labyrinth seals to seal betweenrotating and non-rotating members, however large clearances are requiredat some engine operating conditions to accommodate relative movement. Itis desirable to reduce or alter these clearances at certain operatingconditions to achieve the most effective power output of the engine.

[0006] It is also a requirement of such seals that an acceptableclearance gap is provided at all the differing engine conditions. Suchseals between rotating and non-rotating members are not required toprovide a complete closed seal but are required to provide a seal with apredetermined clearance range.

[0007] Thermal expansion and changes in pressure conditions can alsocause unbalanced forces on the seal and affect the seals effectiveness.It is therefore important that the clearance between the non-rotatingpart of the seal and its static part is kept within a predeterminedrange.

[0008] It is therefore an aim of the present invention to provide animproved seal which may also alleviate the aforementioned problems.

[0009] According to the present invention there is provided a seal forproviding sealing between at least two separate and differing pressurezones and between a rotating structure and non-rotating structures,comprising first and second sealing means, the first sealing meanscomprising first and second seal lands positioned either side of arotating seal member, said seal lands being connected together viaconnecting means, said connecting means being movably mounted on saidnon-rotating structure and arranged to be moveable so as to accommodaterelative movement of said rotating and non-rotating structures, saidsecond seal member being arranged and positioned to provide a sealbetween said non-rotating structure and the first seal land positionedin a lower pressure zone such that the pressure around this seal land iscontrolled.

[0010] The seal lands may comprise two opposing magnets arranged torepel one another.

[0011] Preferably the sealing lands comprise rings.

[0012] Also preferably the sealing member comprises a rotating sealingfin attached to a rotor of a gas turbine engine.

[0013] The connecting means may comprise a yoke.

[0014] The yoke may be connected to the non-rotating member by a firstpivot, allowing rotational movement of the yoke.

[0015] The opposing faces of the seal lands may comprise reduced areaportion positioned opposite one another.

[0016] Embodiments of the present invention are described below withreference to the accompanying drawings in which:

[0017]FIG. 1 is a partially cut-away view of a turbo fan gas turbineengine having one or more seals according to the present invention;

[0018]FIG. 2 is an illustration of the general concept of the presentinvention;

[0019]FIG. 3 is another illustration of a general concept of the presentinvention;

[0020]FIG. 4 is another illustration of the general concept of thepresent invention;

[0021]FIG. 5 is a sectional view through a compressor rotor and adjacentstator incorporating a seal according to a embodiment of the presentinvention;

[0022]FIG. 6 is a view of FIG. 5 illustrating a seal's position in a gasturbine engine; and

[0023]FIG. 7 is a diagrammatic view of a further embodiment of thepresent invention.

[0024]FIG. 8 is a diagrammatic view of another seal according to thepresent invention.

[0025]FIG. 9 is a diagrammatic view of an alternative seal according tothe present invention.

[0026]FIG. 10 is a diagrammatic view of a further seal according to thepresent invention.

[0027]FIG. 11 is a diagrammatic view of an additional seal according tothe present invention.

[0028]FIG. 12 is a diagrammatic view of a final seal according to thepresent invention.

[0029] A gas turbine engine 10 is shown in FIG. 1 and comprises in axialflow series, an intake 12, a fan section 14, an intermediate pressurecompressor section 16, a high pressure compressor section 18, acombustion section 20, a high pressure turbine section 22,, anintermediate pressure turbine section 24, a low pressure turbine section26 and an outlet 28. The fan section 14 has a fan outlet 30 to providebypass flow. The low pressure turbine section 26 is arranged to drivethe fan section 14 by a first shaft 32, the intermediate pressureturbine section 24 is arranged to drive the intermediate pressurecompressor section 16 by a shaft 34 and the high pressure turbinesection 26 is arranged to drive the high pressure compressor section 18by a shaft 36.

[0030] The gas turbine engine 10 operates conventionally in that air iscompressed as it flows through the fan section 14, the intermediatepressure compressor section 16 and the high pressure compressor section18. The air is then delivered into the combustion section 20 and fuel isinjected into the combustion section 20 and burnt in the air to producehot gases. These hot gases flow through and drive the high pressureturbine section 22, the intermediate pressure turbine section 24 and thelow pressure turbine section 26. The hot gases then flow through theoutlet 28 to provide some thrust. However, the main thrust is providedby the air compressed by the fan section 14 and discharged through thefan outlet 30.

[0031] A seal arrangement 40 is described below with respect to a gasturbine engine, although it is to be appreciated that the seal issuitable for any application between relatively moveable componentswhere sealing is required.

[0032]FIGS. 2, 3 and 4 are provided to explain the general concept ofthe present invention. Embodiments of the present invention with respectto practical applications are shown in FIGS. 5, 6 and 7.

[0033] Referring to FIG. 2, the seal arrangement 40 comprises a rotatingmember 42 attached to a support member 44 which supports a sealing fin46. The sealing fin 46 is mounted, in this conceptual arrangement,between two magnetic rings 48 and 50 and the magnetic rings 48, 50 beingarranged to repel one another. These magnetic discs may be of segmentedform to provide the necessary magnetic repulsion and/or mechanicalflexibility. The sealing fin 46 is manufactured from a conductingmaterial and is positioned at the mid point between the two magneticrings 48 and 50. As the sealing fin 46 is manufactured to be thin, noflux is cut between the magnets 48 and 50 and therefore no drag isgenerated at the central position. As the sealing fin 46 is displacedfrom its central position the flux is cut and a restoring force isgenerated. This restoring force centralises the sealing fin 46 betweenthe magnetic rings 48 and 50. The sealing apparatus 40 shown in FIG. 2and FIG. 3 is arranged to provide a seal between a static structure 52which could be envisaged as the casing of a turbine of a gas turbineengine, and the rotating structure 42. The magnetic rings 48 and 50 arerigidly connected together via a yoke 54 which is flexibly attached tothe casing by one or more leaf springs 56 which allow the magnetic rings48, 50 via their yoke 54 to move axially with respect to the rotatingfin 46.

[0034] A number of pressure zones are located around sealing apparatus40, which could be considered to be the high pressure and low pressurezones located within the compressor or turbine of a gas turbine engine.One side of the rotating fin 46 and attached structure is adjacent ahigh pressure zone 60. The other side of the rotating member 42 and itsattached sealing fin 46 is adjacent a low pressure zone 62.

[0035] The relative position between the sealing fin 46 and the staticstructure 52 is altered during normal operation due to factors such asdifferential thermal expansion, centrifugal growth, and changes inpressure. The purpose of the seal is to prevent passage of fluid fromthe high pressure zone 60 to the low pressure zone 62. Forces aregenerated in the moveable segment by pressures around the elements andany bias which may be applied by the leaf springs 56.

[0036] As the forces produced by pressure changes increase, the magneticrestoring forces need to increase to accommodate such changes. If suchforces generated by pressure differences on the seal 40 are reduced thenthe seal 40 can be more responsive and less expensive.

[0037] A second seal 64 is therefore located such that a seal betweenthe high pressure zone 60 and low pressure zone 62 is not only providedby sealing fin 46 but also secondary seal 64. In this conceptual exampleof the present invention a secondary seal 64 comprises a second sealingfin 66 located outwardly from magnetic ring 50, this fin 66 would alsobe formed as a disc. The fin 46 is in sealing contact with secondaryseal 68 which is attached to casing 52.

[0038] The main function of first: sealing fin 46 is to maintain asuitable clearance gap thus accommodating displacement of the staticmember 52 or rotating member 42 in response to pressure changes and ingeneral to any movement between said static member 52 and rotatingmember 42.

[0039] Pressure distribution around the seal may be controlled byeffective positioning of the secondary seal 64. Without such a secondaryseal 64 the main seal 40 is required to provide magnetic forces tocounter large forces provided by pressure differences. The positioningof the secondary seal 64 as shown in FIGS. 2 and 3, provides a highpressure zone around the majority of the seal as shown. The pressure maybe used to balance, or minimise, the forces which are generated. Anoffset force may also be provided by positioning of the secondary seal64. This will have a significant effect on the required size, weight,stability and cost of the magnetic seal lands 48 and 50 and the otherelements of the seal system.

[0040] Full pressure balancing may be obtained by arranging orpositioning the secondary seal such that an area equal to half of themain seal land area is subject to the high pressure. The main seal landarea is indicated by small letter ‘a’ and the secondary seal land areais indicated by small letter ‘f’.

[0041] This concept can be understood more clearly by referring to FIG.4 and by considering the equations set out below.

[0042] The force, F, on magnet 48 is derived as follows:

[0043] The forces on face a, Fa are defined by

Fa=HPa

[0044] HP is high pressure and a is area of face a

[0045] The force on face b, Fb is defined by Fb=HPb, where b is the areaof face b

[0046] The force on face c, Fc is defined by Fc=½(HP+IP)c, where c isthe area of face c and IP is an pressure intermediate high pressure andlow pressure, this is a simplifying assumption that the pressure IP ismidway between pressure HP and pressure IP,

Opposing force=HPb+½(HP+IP)c

[0047] The force on face b is balanced by the force on equivalent partof a and so for this calculation may be disregarded.

[0048] The term ½ (HP+IP) is used to give the average pressure over areac. This is an approximation only, but provides a reasonably good firstorder approximation. The force to the right on magnet 48 is therefore:

F ₁=HPc−½(HP+IP)c

[0049] Similarly the force, F₂, on magnet 50 is

[0050] The force on face e is balanced by the force on face g and may bedisregarded.

F ₂=½(IP+LP)d−fHP−(d−f)LP

[0051] (assuming f<d), c=d

[0052] if a value of f=d/2 is used, then F₁+F₂=0

[0053] i.e. the forces F₁ and F₂ are completely balanced.

[0054] Some forces will be generated by flow effects (Bernoulli etc.)and by imperfections in the balancing. Forces will also be needed toaccelerate movement of the magnets and associated mechanism during rapidtransients.

[0055] The area of pressure lands of the magnets 48 and 50 are reduced,as indicated by c and d and may be used to reduce the effect orimperfections by allowing the forces to act only on smaller areas. Theseal still operates without the reduced land i.e. c=a, b=o, but higherforces may be generated, leading to the need for higher magnetic forces,size, weight, etc. The same principle may be used with other forcegenerating mechanisms.

[0056] Thus it can be appreciated that changes in pressures as the yoke54 moves axially, will produce forces which restore it to a positionwhere equal gaps are achieved on either side of the sealing fin 46 thusproviding a fully pressure balanced seal.

[0057] A practical application of the concept indicated in FIGS. 2, 3and 4 is shown in FIGS. 5 and 6. In this embodiment of the invention aseal 70 similar to the conceptual arrangement shown in FIGS. 2, 3 and 4is arranged to provide a seal between the fan rotor 71 and adjacentstator structure 73 of a gas turbine engine. The downstream end of thefan rotor is provided with a sealing fin 74 attached thereto by a bolt76. This sealing fin 74 comprises a ring structure and is mountedradially between two ring magnets 77, 79. The sealing fin 74 being thinso as to provide a null-flux magnetic zone around the central regionbetween ring magnets 77 and 79. The ring magnets 77, 79 are rigidlymounted on a yoke 78. The yoke 78 comprises a U shaped cut away portion81 into which the sealing fin disc 74 protrudes. The yoke 78 is pivotalymounted on the stator by a pivot point 80. This pivot point 80 allowsrotational movement of said yoke and hence can control the sealingclearance 82 between said ring magnets 77, 79 and said sealing fin 74.

[0058] A secondary seal 84 is provided, and this secondary seal 84comprises a hook type protrusion 86 on said stator structure 73 and aco-operating seal land 88 mounted on said yoke 78.

[0059] The positioning of this secondary seal 84 prevents low pressureair indicated in FIGS. 5, 6 and 7 by LP, flowing freely around the rear90 of seal 70 thus allowing the rear of the seal to be provided withinhigh pressure zone, HP. An intermediate pressure zone, IP, is formedaround the downstream end 92 of the sealing fin 74.

[0060] The secondary seal 84 is positioned to follow an arc around thepivot point 80. This position of the secondary seal 84 is chosen tobalance the rotational forces generated by the pressures around thepivot point 80. Using the pressure balance equation previously noted inthis specification, it is clear that pressure balance can be obtained byarranging the secondary seal 84 to be provided with a seal land areacalculated for pressure balancing.

[0061]FIG. 6 is a view of a seal according to the present invention,mounted within a gas turbine engine. A fan blade 95 is mounted on a fandisc 97 and seal apparatus 70 seals between the high pressure and lowpressure zones.

[0062] Conventionally in magnetic levitating seals or air riding sealsetc. a gap closing force is applied which is opposed by levitation. Thisrequires large forces to be generated or maintained to overcome thisclosing force. The present invention reduces this closing force whichallows the seal to be smaller, lighter and be provided by a lower coststructure. Such a seal arrangement also provides a possibility for afaster response to movements of the sealing structures. The presentinvention arranges two seals to operate in opposition with any pressureforces being totally or partially balanced.

[0063]FIG. 7 illustrates the further embodiment of the present inventionwhereby the magnets are replaced by an air riding seal system 94. Againthis air riding seal 94 is provided to seal between a rotating rotor 96comprising a sealing fin 98 extending there from and two halves 100 and102 of the air riding seal 94, and a static casing 104. In thisembodiment of the invention the two halves 100, 102 of the seal 94 areseparated as the engine stops, thus preventing contact as the centringforces are lost. The two halves 100 and 102 are rings. Secondary seal106 provides the pressure balancing as previously explained. The twohalves 100, 102 of the seal system 94 are pushed apart by one or moresprings 112, are located on locating pins 108 and are centralised by twosets of weaker springs 110. In operation, as the pressure increases andthe engine is started, the two halves 100 and 102 are pushed togetheruntil they reach their normal operating clearance. At this stage, theymove together as a pressure balance seal with additional centring forcesprovided by the air riding seal mechanism. It is envisaged that othermechanisms such as temperature activated, or bi-metallic structurescould be provided to achieve retraction of the seals when not required.

[0064] Another embodiment of the present invention is shown in FIG. 8and this illustrates an air riding seal system 94B similar to that shownin FIG. 7. The air riding seal 94 is provided to seal between a rotor 96comprising a sealing fin 98 extending therefrom, two interconnectedhalves 100 and 102 of the air riding seal 94 and a stator structure 104.The air riding seal 94 is mounted on the static structure 104 by leafsprings 103. The two halves 100 and 102 are rings. The secondary seal106B provides the pressure balancing as previously explained. Thesecondary seal 106 is also an air riding seal. A passage 105 through thestatic structure 104 supplies, in operation, high pressure air to thesecondary seal 106B to form an air riding seal/air bearing. The forcesare essentially balanced and hence the air riding seal 94 may beoperated with relatively large clearance to minimise change from debris.

[0065] An alternative embodiment of the present invention is shown inFIG. 9 wherein the magnets are replaced by a brush seal system 120 andis similar to that shown in FIG. 5. The brush seal 120 is provided toseal between a rotor 71 and static structure 73. A sealing fin 74 isattached to the rotor 71. The sealing fin 74 is mounted between twobrush seals 122 and 124. The brush seals 122 and 124 are rigidly mountedon a yoke 78, the yoke 78 comprises a U shaped cut away portion 81 intowhich the sealing fin 74 extends. The yoke 78 is pivotally mounted onthe stator structure 73 at a pivot point 80. The pivot point 80 allowsrotational movement of the yoke 78 and hence can control the sealingclearance 82 between the brush seals 122 and 124 and the sealing fin 74.A secondary seal 84 is provided and comprises a protrusion 86 on thestatic structure 73 and a cooperating seal land 88 on the yoke 78. Thebrush seals 122 and 124 are annular. The sealing fin 78 may extendradially and the brush seals 122 and 124 extend axially or the sealingfin 74 may extend radially and the brush seals 122 and 124 extendradially.

[0066] A further embodiment of the present invention is shown in FIG. 10and this illustrates a labyrinth seal system 130. The labyrinth seal 130is provided to seal between a turbine rotor 71 comprising an axiallyextending annular fin 74 and a stator structure 73. A yoke 78 is mountedon the static structure 73 by leaf springs 75, and the yoke 78 comprisesa U shaped cut away portion into which the sealing fin 74 extends. Theleaf springs 75 allow radial movement of the yoke 78 and hence controlthe sealing fin 74 and the yoke 78. The sealing fin 74 has projections134 extending radially outwardly and projections 132 extending radiallyinwardly which cooperate with abradable coatings 138 and 136respectively on the surfaces of the yoke 78.

[0067] An additional embodiment of the present invention is shown inFIG. 11 and this illustrates a magnetic seal system 140. The magneticseal system 140 is similar to that shown in FIGS. 2, 3 and 4. Themagnetic seal system 140 differs in that the annular side wall 55 of theyoke 54, closest to the secondary seal 68 attached to the casing 52 isperforated whereas the annular side wall 53 of the yoke 54, furthestfrom the secondary seal 68 attached to the casing 52, is not perforated.The perforated annular side wall 55 of the yoke 54 has a smaller surfacearea than the unperforated annular side wall 53 of the yoke 54. A thirdseal 65 is provided such that the intermediate pressure IP zone isdefined by the yoke 54, the second seal 64 and the third seal 65. Thethird seal 65 comprises a third annular sealing fin 67 extending fromthe yoke 54 from a position between the side walls 53 and 55 of the yoke54, towards but spaced from the second seal member 68 and an annularhook shaped projection 69 extending from the second seal member 68towards but spaced from the third annular sealing fin 67 to form thethird seal 65. The difference in surface area between the side walls 53and 55 of the yoke 54 results in the magnets 48 and 50 and yoke 54initially moving towards the left until the rise in intermediatepressure moves the magnets 48 and 50 and yoke 54 towards the right andback to the balance position. The second seal 64 is positioned radiallyto balance the loads on the seal 140 at the central position.

[0068] A further embodiment of the present invention is shown in FIG. 12and this illustrates a magnetic seal system 150. The magnetic sealsystem 150 is substantially the same as that shown in FIGS. 2, 3 and 4.The magnetic seal system 150 differs in that the tip 47 of the sealingfin 46 is shaped to increase the aerodynamic lift between the sealingfin 46 and the sealing lands 48 and 50. The tip 47 of the sealing fin 46is located substantially between the magnets 48 and 50. The sealing fin46 has a first surface 46A facing the high pressure zone 60 and a secondsurface 46B facing the low pressure zone 62. The tip 47 of the sealingfin 46 is stepped such that there is a curved surface portion 47Ainterconnecting surface 46A with a planar surface portion 47B, which isparallel to surface 46A, and a curved surface portion 47Cinterconnecting surface portion 47B with a planar surface portion 47D,which is parallel to surface 46A and surface portion 47B. Surfaceportion 47B is nearer to the magnet 48 than surface 46A and surfaceportion 47D is nearer to the magnet 48 than surface portions 47B.Similarly there is a curved surface portion 47E interconnecting surface46B with a planar surface portion 47F, which is parallel to surface 46B,and a curved surface portion 47G interconnecting surface portion 47Fwith a planar surface portion 47H, which is parallel to surface 46B andsurface portion 47F. Surface portion 47F is further from the magnet 50than surface 46B and surface portion 47H is further from the magnet 50than surface portion 47F.

[0069] In operation fluid flows from the high pressure zone 60, to thelow pressure zone 62 through the seal 150. The fluid initially flows inthe direction of arrows A along the surface 46A of the sealing fin 46towards the tip 47 of the sealing fin 46. The fluid is directed to flowaway from the surface 46A by the curved surface portion 47A of the tip47 towards the magnet 48. Additionally the curved surface portion 47C ofthe tip 47 directs the fluid away from the surface portion 47B towardsthe yoke portion 54A. This directing of fluid flow towards the magnet 48increases the lift between the sealing fin 46 and the magnet 48 and yoke54. Similarly the fluid flows in the direction of arrows B along thesurface 47H of the tip 47 of the sealing fin 46 towards the surface 46Bof the sealing fin 46. The fluid is directed to flow away from thesurface 47H by the curved surface portion 47G of the tip 47 towards themagnet 50. Additionally the curved surface portion 47E of the tip 47directs the fluid away from the surface portion 47F towards the yokeportion 54B. This directing of fluid towards the magnet 50 increases thelift between the sealing fin 46 and the magnet 50/yoke 54. This isbecause the fluid velocity in a small clearance gap is greater than thefluid velocity in a large clearance gap for the same mass flow and hencethere is a greater force with a small clearance gap. The clearance gapis of the order of 80 μm in width and 10 mm long.

[0070] Although the present invention has described a stepped sealingfin for the magnetic seal it is equally possible to apply the principleto an air riding seal, brush seal and labyrinth seal.

[0071] Although the present invention has described the use of spring,or pivot, mounting of the connection between the seal lands to thestatic structure other suitable methods may be used.

[0072] Whilst endeavouring in the foregoing specification to drawattention to those features of the invention believed to be ofparticular importance it should be understood that the Applicant claimsprotection in respect of any patentable feature or combination. offeatures hereinbefore referred to and/or shown in the drawings whetheror not particular emphasis has been placed thereon.

I claim:
 1. A seal for providing sealing between at least two separateand differing pressure zones and between a rotating structure andnon-rotating structures, comprising first and second sealing means, thefirst sealing means comprising first and second seal lands positionedeither side of a rotating seal member, said seal lands being connectedtogether via connecting means, said connecting means being movablymounted on said non-rotating structure and arranged to be moveable so asto accommodate relative movement of said rotating and non-rotatingstructures, said second seal means being arranged and positioned toprovide a seal between said non-rotating structure and the first sealland positioned in a lower pressure zone such that the pressure aroundthis seal land is controlled.
 2. A seal as claim in claim 1 wherein thetwo seal lands comprise two opposing magnets arranged to repel oneanother.
 3. A seal as claimed in claim 2 wherein the magnetic sealinglands comprise rings.
 4. A seal as claimed in claim 3 wherein the ringscomprise segmented magnetic rings.
 5. A seal as claimed in claim 4wherein seals are provided between the segments of the magnetic rings.6. A seal as claimed in claim 2 wherein said rotating seal membercomprises a rotating sealing disc of a conducting material.
 7. A seal asclaimed in claim 6 wherein said rotating sealing disc is located in anintermediate pressure zone.
 8. A seal as claimed in claim 1 wherein saidfirst sealing means comprises an air riding seal.
 9. A seal as claimedin claim 8 wherein said air riding seal comprises two rings arranged onopposite sides of the rotating seal member.
 10. A seal as claimed inclaim 9 wherein first biasing means is provided to bias the two ringsapart and second biasing means to bias the two rings together.
 11. Aseal as claimed in claim 10 wherein the first and second biasing meanscomprise springs.
 12. A seal as claimed in claim 1 wherein said firstsealing means comprises a brush seal.
 13. A seal as claimed in claim 12wherein the brush seal comprises two brush seals arranged on oppositesides of the rotating seal member.
 14. A seal as claimed in claim 1wherein said first sealing means comprises a labyrinth seal.
 15. A sealas claimed in claim 14 wherein the labyrinth seal comprises twolabyrinth seals arranged on opposite sides of the rotating seal member.16. A seal as claimed in claim 1 wherein the opposing faces of said seallands each comprise a reduced area portion.
 17. A seal as claimed inclaim 16 wherein the reduced area portion of said seal lands arepositioned directly opposite one another.
 18. A seal as claimed inclaims 16 wherein the second seal member is arranged and positioned suchthat the surface areas of the reduced area portions of the opposingfaces of the seal land, are equal and the surface area of a portion of aface on the first seal land remote from the opposing faces of the seallands is half of the surface area of the reduced area portion of theopposing face of the first seal land such that the forces on the seallands are substantially balanced.
 19. A seal as claimed in claim 1comprising a third seal sealing means, the third sealing means beingarranged and positioned to provide a seal between the non rotatingstructure and the connecting means, the connecting means between thesecond sealing means and the third sealing means being perforated suchthat the chamber partially defined by the second sealing means and thirdsealing means is interconnected with the chamber defined by the firstand second seal lands and the connecting means.
 20. A seal as claimed inclaim 1 wherein the rotating sealing member is shaped to increase theaerodynamic lift between the rotating sealing member and the first andsecond sealing lands.
 21. A seal as claimed in claim 20 wherein therotating sealing member has curved surface portions to direct fluidtowards the first and second sealing lands.
 22. A seal as claimed inclaim 20 wherein the rotating sealing member has curved surface portionsto direct fluid towards the connecting means.
 23. A seal as claimed inclaim 1 wherein said connecting means, comprises a yoke.
 24. A seal asclaimed in claim 23 wherein the yoke is connected to the non-rotatingmember by one or more springs.
 25. A seal as claimed in claim 23 whereinsaid yoke is connected to said non-rotating member by a pivot pointwhich allows rotational movement of said yoke.
 26. A seal as claimed inclaim 1 wherein said rotating sealing member comprises a rotatingsealing fin attached to a rotor of a gas turbine engine and said nonrotating structure comprises an adjacent static structure of the gasturbine engine.
 27. A seal as claimed in claim 26 wherein the rotor is acompressor rotor.